I think people would appreciate an all-black version, like the C421. I've seen a few people disappointed by the lack of an all-black option, for the time being, and given the current choices of colours they would pick the silver one. Black offers a sleeker, more low-profile/conservative kind of look in my honest opinion, as black coloured electronics are quite commonly found.
I wonder how popular the slate option will be. It certainly looks nice, don't get me wrong, but it's not a typical colour offered in electronics.
Most portable amps on the market these days are either silver or black. What's wrong with a little variety
As someone looking for a subtle amp to pair with iDevices (and hopefully the new Fiio X3), the slate seems perfect.
Oh nothing is wrong with variety, it's just odd to some people that the "industry standard" black colour isn't offered, yet.